This invention relates to a lamp lighting apparatus and a projector which uses the lamp lighting apparatus, and more particularly to a lamp lighting apparatus which uses a metal halide lamp or the like and a projector which uses the lamp lighting apparatus.
A lamp lighting apparatus generates a high voltage for lighting and applies the high voltage to both of electrodes of a lamp to cause glow discharge to be produced by a glow switch or the like provided in the lamp. Then, the glow discharge changes into arc discharge in an arc tube to light the lamp.
A typical lamp lighting apparatus shown in FIG. 8 includes an active filter, for example. Referring to FIG. 8, the lamp lighting apparatus comprises a down converter 52 which receives a DC voltage E1 normally of approximately 300 to 400 Vdc from a DC power supply and converts the DC voltage E1 once into a DC voltage V1 of approximately 50 to 100 Vdc, a controller 53 which inputs a control signal C1 obtained by comparing a detection power W1 from a power detection section 59 hereinafter described and a reference power W2 with each other to the down converter 52 to control the DC voltage V1 to keep a fixed power, a full bridge 54 which converts the DC voltage V1 of approximately 50 to 100 Vdc into an AC current having a frequency of approximately 90 to 200 Hz which is necessary to keep discharge of a lamp 62 and supplies the resulting AC current to the lamp 62, a controller 55 which sets a reference frequency fs of a drive signal for driving the full bridge 54 and controls on/off of the drive signal, an igniter 56 including an ignition outputting transformer T2 which generates a high voltage of 5 to 20 kV for lighting of the lamp 62 and transmits an AC voltage V2 of approximately 50 to 100 V for keeping of arc discharge, a voltage control section 60 for controlling the igniter 56, a voltage detection section 57 which detects a voltage value of the AC voltage V2 to be supplied from the full bridge 54 to the igniter 56, a current detection section 59 which detects a current value of AC current I2 to be supplied from the full bridge 54 to the igniter 56, a power detection section 58 which detects a detection power W1 from the voltage detected by the voltage detection section 57 and the current detected by the current detection section 58, and a connector 61 for establishing a connection to the lamp 62. It is to be noted that, when the present lamp lighting apparatus is applied to a projector, the controller 53 can be used also as a system controller, and if a power switch not shown is depressed and the controller 53 discriminates a power supply off state, then the controller 53, for example, opens a contact of a relay to switch off supply of the DC voltage E1 and enters a standby state. Then, if the power switch is depressed again and the controller 53 discriminates a power supply on state, then the present lamp lighting apparatus enters a normal operation mode and closes the contact of the relay to supply the DC voltage E1.
The voltage control section 60 performs control for producing a high voltage of 5 to 20 kV upon lighting of the lamp 62. An exemplary configuration of the voltage control section 60 is shown in FIG. 9. Referring to FIG. 9, the voltage control section 60 shown includes a resister R31 and a capacitor C31 which form a series circuit to which a DC power supply DC of 300 Vdc is supplied, a boosting transformer T31 connected to a node between the resister R31 and the capacitor C31 through a switching element H31 such as a SIDAC, a diode D31 and a discharging gap element H32 which are connected to a secondary winding N2 of the boosting transformer T31 and form a series circuit, and a capacitor C32 which is connected to a node between the diode D31 and the discharging gap element H32 and which forms a parallel circuit together with the secondary winding N2 of the boosting transformer T31. The output side of the discharging gap element H32 is connected to a terminal T5, and the output side of the secondary winding N2 of the boosting transformer T31 and the capacitor C32 which form the parallel circuit is connected to a terminal T6.
Referring back to FIG. 8, the full bridge 54 is formed from a full bridge including, for example, field effect transistors (FETs) or the like. The gates of the FETs are controlled between on and off based on the reference frequency fs set by the controller 55.
Consequently, the full bridge 54 can convert the DC voltage V1 into an AC current having a frequency of approximately 90 to 200 Hz and can supply an AC current I2 necessary to keep lighting of the lamp 62 to the lamp 62 through the igniter 56 and the connector 61 which has a Lo terminal and a Hi terminal.
Referring to FIGS. 8 and 9, in the voltage control section 60 having the connection scheme described above, a voltage of 300 Vdc of the DC power supply first charges the capacitor C31 through the resister R31. Then, if the charging voltage of the capacitor C31 reaches, for example, 200 V, then the switching device H31 enters into a conducting state, and as a result, an excitation current flows to the primary side of the boosting transformer T31 while the capacitor C31 discharges. The charging voltage of the capacitor C31 drops as a result of the discharge just described, and finally, the switching device H31 enters into a non-conducting state and the excitation current does not flow to the boosting transformer T31 any more. Then, the voltage from the DC power supply charges the capacitor C31 again through the resister R31.
By the repeating cycle described above, a pulse voltage raised to 2 to 3 kV can be repetitively obtained on the secondary side of the boosting transformer T31.
The pulse voltage repetitively charges the capacitor C32 through the diode D31 on the secondary side of the boosting transformer T31, and as a result, a charging voltage of the capacitor C32 gradually rises. If this charging voltage reaches, for example, 1 kV, then the discharging gap element H32 starts discharge and an excitation current flows to the primary side of the outputting transformer T2 of the igniter 56 while the capacitor C32 discharges. The charging voltage drops as the capacitor C32 discharges, and finally, the discharging gap element H32 stops the discharge and the excitation current does not flow to the outputting transformer T2. Then, the raised pulse voltage charges the capacitor C32 again through the diode D31.
By the repeating cycle described above, a pulse voltage raised to, for example, 5 kV can be obtained on the secondary side of the outputting transformer T2 and the lamp 62 is lit.
The lamp 62 may be, for example, a discharge lamp 11 such as a metal halide lamp shown in FIG. 10. Referring to FIG. 10, the discharge lamp 11 has a pair of electrodes 26 and 27 of the same structure arranged in a spaced relationship from each other in a translucent airtight vessel 25 formed from, for example, a heat resisting glass material so that they may have a predetermined electrode space distance L therebetween. The electrodes 26 and 27 are driven with an AC current.
The electrodes 26 and 27 are connected to metal conductors 30 and 31 sealed in seal sections 28 and 29 formed at both ends of the airtight vessel 25. The metal conductor 30 is connected to a terminal Hi of a connector 17 and the other metal conductor 31 is connected to another terminal Lo. Further, the seal section 28 of the airtight vessel 25 is fixed to a central portion of a reflector 32 having a hemispherical shape.
Then, a discharging medium which includes, for example, rare gas, cesium, rare earth metal, and halogen in addition to mercury and wherein the encapsulated amount of cesium is within a predetermined range is encapsulated in the airtight vessel 25. Thus, since steam of the several kinds of metals is included in a discharging arc of the mercury steam, peculiar optical spectra to the metals are emitted.
Consequently, the light emission efficiency is high, and a desired light emission characteristic can be obtained depending upon a combination of metals. Further, also the color temperature is high, and also the color is near to that of the natural light and a point-source light and parallel light can be extracted readily using a reflector or the like. Due to the advantages just described, the lamp 62 is utilized for a liquid crystal projector for which a high picture quality and a high luminance are required.
However, in the high voltage discharging lamp having such a configuration as described above, the discharge starting voltage is as low as, for example, 5 kV in a state wherein the temperature is low because of lapse of time after it is extinguished. However, in another state wherein the temperature of the lamp is high as at an instance such as immediately after it is extinguished, since the discharge starting voltage is high because the steam pressure is high, the lamp cannot be lit again with a voltage equal to that in the state wherein the temperature of the lamp is low. Only after the temperature drops as the time elapses and the steam pressure drops, discharging becomes possible and the lamp is lit again.
In this manner, in order to make it possible for the lamp to be lit even in a state wherein the temperature thereof is high immediately after it is extinguished, an unnecessarily high voltage is applied to the lamp.
Further, in the voltage control section 60 which performs ignition as described above with reference to FIG. 9, since discharge is started when the charging voltage of the capacitor C32 reaches the discharge starting voltage of the discharging gap element H32, the lighting voltage of the lamp depends upon the characteristic of the discharging gap element H32. Therefore, the voltage control section 60 of the lamp lighting apparatus has a drawback in that the discharging gap element H32 must be selected suitably for the lamp.
Accordingly, it is demanded to provide a lamp lighting apparatus which can light a lamp any time without the necessity for selection of a discharging gap element and without application of an unnecessarily high voltage.
In order to attain the object described above, according to an aspect of the present invention, there is provided a lamp lighting apparatus comprising voltage controlling means for supplying a voltage whose peak value rises stepwise, and power conversion means for converting an output of the voltage controlling means into a high voltage and applying the high voltage to a lamp to light the lamp and for supplying, after the lamp is lit, an AC current necessary to keep discharge of the lamp to the lamp.
In the lamp lighting apparatus, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, whatever characteristic the lamp has, the lamp can be lit with a necessary but minimum high pulse voltage conforming with the characteristic of the lamp. Consequently, the burden on the lamp can be moderated and the life of the lamp can be increased.
Further, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, only a minimum voltage is applied to a winding of the power conversion means, and consequently, otherwise possible deterioration by a high voltage can be prevented.
Furthermore, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, the lamp can be replaced by another lamp which may have any different discharge starting voltage. This augments the convenience in use and makes the process of manufacture flexible. Also augmentation in service can be anticipated.
In addition, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, when the lamp has a low temperature because time has passed after it was extinguished last, it can be lit with a low lighting voltage, but when the same lamp has a high temperature because much time has not passed after it was extinguished last, it can be lit with a corresponding high voltage. Consequently, the convenience in use can be augmented.
Where the voltage controlling means includes a charging circuit for supplying the charging voltage, a voltage dividing circuit for dividing the output of the charging circuit and a circuit for lowering the dividing ratio of the voltage dividing circuit stepwise, since the output of the voltage dividing circuit increases the time required until the predetermined voltage is reached, the peak value of the charging voltage can be increased stepwise.
According to another aspect of the present invention, there is provided a projector comprising voltage controlling means for supplying a voltage whose peak value rises stepwise, power conversion means for converting an output of the voltage controlling means into a high voltage and applying the high voltage to a lamp to light the lamp and for supplying, after the lamp is lit, an AC current necessary to keep discharge of the lamp to the lamp to keep the lamp in the lit state, a display panel for transmitting light therethrough or reflecting light emitted from the lamp lit by the power conversion means, and an optical system for projecting the output light of the display panel.
In the projector, a high pulse voltage whose peak value is controlled to rise stepwise by the voltage controlling means is supplied to the lamp, even when the lamp is not sufficiently cooled immediately after the power supply to the projector is stopped, the power supply can be made available to light the lamp. Consequently, the projector is augmented in convenience in use.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.